[TSan][MIPS] Expand sanitizer memory space to lower addresses

MemToShadowImpl() maps lower addresses to a memory space out of sanitizers
range. The simplest example is address 0 which is mapped to 0x2000000000

static const uptr kShadowBeg     = 0x2400000000ull;

but accessing the address during tsan execution will lead to a segmentation
fault.

This patch expands the range used by the sanitizer and ensures that 1/8 of
the maximum valid address in the virtual address spaces is used for shadow
memory.

Patch by Milos Stojanovic.

Differential Revision: https://reviews.llvm.org/D41777

llvm-svn: 323013